72 
MUTUAL CHECKS TO INCREASE. 
Chap. III. 
When I ascertained that these young trees had not 
been sown or planted, I was so much surprised at their 
numbers that I went to several points of view, whence 
I could examine hundreds of acres of the unenclosed 
heath, and literally I could not see a single Scotch fir, 
except the old planted clumps. But on looking closely 
between the stems of the heath, I found a multitude of 
seedlings and little trees, which had been perpetually 
brow^sed down by the cattle. In one square yard, at a 
point some hundred yards distant from one of the old 
clumps, I counted thirty-two little trees; and one of 
them, with twenty-six rings of growth, had during many 
vears tried to raise its head above the stems of the 
heath, and had failed. No wonder that, as soon as the 
land was enclosed, it became thickly clothed with vigor¬ 
ously growing young firs. Yet the heath was so ex¬ 
tremely barren and so extensive that no one would ever 
have imagined that cattle would have so closely and 
effectually searched it for food. 
Here we see that cattle absolutely determine the 
existence of the Scotch fir; but in several parts of the 
world insects determine the existence of cattle. Perhaps 
Paraguay offers the most curious instance of this; for 
here neither cattle nor horses nor dogs have ever run 
wild, though they swarm southward and northward in a 
feral state; and Azara and Eengger have shown that 
this is caused by the greater number in Paraguay of a 
certain fiy, which lays its eggs in the navels of these 
animals when first born. The increase of these fiies, 
numerous as they are, must be habitually checked by 
some means, probably by birds. Hence, if certain in¬ 
sectivorous birds (whose numbers are probably regulated 
by hawks or beasts of prey) were to increase in Para¬ 
guay, the flies would decrease—then cattle and horses 
would became feral, and this would certainly greatly 
